vhost: support config change slave message

This msg is used to notify qemu that should get the config of backend.

For example, vhost-user-blk uses this msg to notify guest OS the
capacity of backend has changed.

The need_reply flag is not mandatory because it will block the sender
thread and master process will send get_config message to fetch the
configuration, this need an extra thread to process the vhost message.

Signed-off-by: Li Feng <fengli@smartx.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>